1. Technical Field of the Invention
The present invention relates to the inspection of structures on an object and, more specifically, to a system and method for optically inspecting structures on an object.
2. Description of Related Art
An example of a useful application of the present invention is the inspection of semiconductor wafers (wafers) during and after the manufacturing process. In order to transform a wafer into a microprocessor or other electronic device, the wafer undergoes several manufacturing steps. First, a wafer is cut from a crystal ingot (such as crystallized silicon), and an expitaxial layer (a single layer of silicon crystals) is then grown thereon. The creation of an expitaxial layer is often followed by the growth of high quality oxides on the wafer surface, in a process called oxidation. Next, the wafer undergoes several fabrication steps. Each fabrication step places a layer of ions, or other materials, in the wafer or on the wafer in a predetermined geometric pattern so as to form a portion of an electronic circuit. When the fabrication steps are completed, the wafer surface typically possesses several functional microelectronic devices separated into a plurality of dice.
Common fabrication steps include chemical vapor depositions (CVD), plasma vapor depositions (PVD), etches, ion implantations, diffusions, metalizations, or the growth of structures directly on the wafer. The successful completion of each of these fabrication steps depends largely on the ability to precisely control the geometric placement of gasses, ions, metals, or other deposition materials through etching, implanting, etc., with sub-micron precision. The precise placement of ions, metals, gasses, or other deposits is achieved through a process called photolithography. Though photolithography is well known in the art of microelectronic device manufacturing, a brief description of photolithography is provided so that the reader may more clearly understand the invention.
Photolithography is a pre-fabrication process in which a pattern is transferred from a negative known as a mask (also called a retical) onto a wafer using a technique similar to film development in photography. Photoresist contains photoactive sensitizers, and in a step called imaging, light or other activating illumination produces a chemical change in the photoresist exposed to the light. The first step in photolithography is the preparation of the wafer. The wafer is cleaned, and a liquid photoresist is distributed evenly across the top surface of the wafer. The photoresist is dried, and then the wafer with the photoresist thereon is heated to vaporize any solvents. The mask""s pattern, like a photographic negative, is projected onto one portion of the wafer at a time by a precision optical device known as a xe2x80x9cstepperxe2x80x9d, and is preserved on the wafer by the photoresist. Each area of the wafer that is to become a separate device is called a xe2x80x9cdiexe2x80x9d. After illumination with the mask""s pattern, a solvent washes away either the exposed portions of xe2x80x9cpositivexe2x80x9d photoresist, or another solvent washes away the unexposed portions of xe2x80x9cnegativexe2x80x9d photoresist. This process, called development, leaves the geometric pattern of the mask, or its negative, on each die.
When fabrication is complete, the electrical characteristics of each die are tested. Based on the results of these tests, the die are xe2x80x9cbinnedxe2x80x9d (i.e., they are classified as good or defective). The wafer is then sliced into separate dice which are sorted to discard defective ones. The good dice are then prepared for packaging.
During the imaging step, the photoresist is exposed to a light source at a predetermined wavelength. So called xe2x80x9cpositivexe2x80x9d photoresist is made soluble by exposure to light, such that the area of the photoresist exposed to light (or other illumination) washes away from the wafer when the photoresist is rinsed with a predetermined solvent. The result is that a direct duplicate image of the mask is left on the semiconductor wafer in the form of xe2x80x9cphotoresist structuresxe2x80x9d which constitute a developed photoresist layer. Though various types of photoresist are available, because so-called xe2x80x9cpositivexe2x80x9d photoresist is preferred for small devices, the remaining discussion will address the use of positive photoresist (hereinafter xe2x80x9cphotoresistxe2x80x9d).
By covering portions of the semiconductor wafer with photoresist structures, the entire wafer can, in a fabrication step, be exposed to various chemicals, ions, metals, or etchings without affecting the entire areas under the photoresist structures. After the fabrication step has been completed, a wash step is executed. In the wash step, the remaining photoresist is washed away and the wafer is cleaned. Should another fabrication step be desired, the wafer may undergo another photolithography process. Accordingly, in order to correctly manufacture microelectronic devices, geometrically correct patterns of photoresist structures must be deposited either on or in the wafer during fabrication. And, correct geometric patterning is dependent upon properly imaging and developing photoresist layers.
Each fabrication step is expensive, and adds significant value to the wafer. Furthermore, fabrication steps are difficult and costly to reverse. By contrast, photoresist structures can be removed quickly and with minimal disturbance to the underlying wafer structures. Thus, it is desirable to detect defects in the developed photoresist prior to the fabrication step. Defects are those anomalies which impair or alter electrical characteristics of the die when fabrication is complete, causing the die to be discarded. If a defect is detected in the developed photoresist layer, one simply washes away the photoresist structures and applies another photoresist layer in place of the defective photoresist layer. The most common method used to detect imperfections in a developed photoresist layer is optical inspection. Electronic, ion beam, or X-ray imaging is also available; however, because these imaging techniques illuminate and reconstruct only one point at a time, they are slower and more expensive than optical inspection. Laser imaging techniques capture and compare the angle of reflection of laser beams; however, they lack precision in reporting the position of defects.
Optical inspection devices typically employ a wafer support which holds a wafer under an overhead camera and multiple light sources. In operation, the optical inspection device lights the wafer from several directions in order to fully illuminate the wafer, and the overhead camera captures a gray-scale (black-and-white) image of the wafer with a developed photoresist layer thereon. In a process commonly called convolution, this image is then sent to a computer which compares the image, pixel by pixel, to a stored image of a wafer with a properly constructed photoresist layer thereon. If any differences between the captured image and the image of the wafer with a properly constructed photoresist layer are detected, the computer has detected a defective photoresist layer.
The existing method of optically inspecting a developed photoresist layer has several disadvantages. First, because the information for each pixel must be stored in memory, the present method of optical inspection requires a large amount of memory. Furthermore, since existing optical inspection devices compare images pixel by pixel, they are very slow.
In order to overcome the disadvantages of the existing methods of inspecting photoresist, it would be advantageous to have a system and method of optically inspecting a developed photoresist layer for defects, such as alignment errors, missing photoresist structures, contamination, and skewed photoresist. Ideally, the photoresist defect detection system should be able to reliably detect and assess the location of photoresist structures, such as photoresist islands, which constitute a developed photoresist layer. In addition, it would also be advantageous for such a system to operate more quickly than convolution techniques. The present invention, when utilized for semiconductor inspection, provides such a system and method.
The present invention is a system for inspecting structures on an object. The system comprises a support which supports the object, a device which emits light or energy on a surface of the object to illuminate the surface and the structures thereon, an image capturing device mounted adjacent the support for acquiring an image of the surface of the object or a portion thereof, and a computer which generates a set of high level descriptors referred to as structure grammar from the image. Structure grammar includes the high level descriptors for a particular structure that have common characteristics including relevant features such as size, shape, color, sharpness, and the like.
The computer compares the structure grammar generated from the image to a structure grammar for a defect-free object (or a defect-free portion of the object) and records any differences in the two grammars. Thus, the defect-free object grammar may be generated from computer aided drafting or from an image of a defect-free object. The computer also performs Automatic Defect Classification (ADC). In addition, a knowledge base accessible by the computer receives and stores structure grammar generated from defect images.
The device which emits light or energy on a surface of the object may emit, for example, laser light, X-rays, ion beams, electrons, or light in the infrared, ultraviolet, or visible spectrum. In addition, the image capturing device could be a line scan or an area scan camera.
In another aspect, the present invention is a method of inspecting an object with a structure thereon which has a surface and at least one edge of the surface. The method includes the steps of shining a light or energy on the object having the structure thereon (the light illuminating the surface so as to clearly illuminate the edges of the structure), capturing an image of the object or a portion thereof, producing a set of primitives for the edges of the structure, and comparing the primitives to a corresponding set of primitives generated from a defect-free object or portion thereof.
The method of inspecting a structure may accommodate structures which have a plurality of edges. In this case, the method includes the steps of scanning the image to detect the structure, tracing the structure to produce primitives for the plurality of edges, producing, for the inspected structure, a set of high level descriptors referred to as inspection grammar, and comparing the inspection grammar to a defect-free grammar representative of a defect-free structure or portion thereof.
Depending on the type of image-capturing device utilized, the method may also include the step of converting the image into a pixel-based image. The defect-free grammar may be created from a computer aided design description of the structure, or by processing an image of a defect-free structure.
The method continues by storing the defect-free grammar in a knowledge base, and storing an inspection grammar in a knowledge base as a defect grammar when the inspection grammar differs from the defect-free grammar. The defect grammars are then organized by defect type. The method continues by performing Automatic Defect Classification on the production grammar.